The present invention relates to a current limiting molded case circuit breaker (MCCB) comprising a rotary contact assembly having single or plurality of contact arms. Circuit breakers of this type are commonly used to protect electrical systems whenever abnormalities occur in the system and are well known in the art. These types of breakers utilize a magnetic repulsion force generated between the stationary and moveable contacts during a short-circuit condition to quickly open the circuit breaker contact arms and separate the stationary and moveable contacts. This separation of the contacts interrupts the current flow through the circuit. One problem often encountered with this type of arrangement is that the contact arm may develop a high velocity due to the large magnetic forces, thus causing it to rebound off the contact arm stop surface. Unless this rebounding is taken into consideration, the contact arm will reclose and allow high levels of current to flow once again into the system. This reclosing action also releases a large amount of energy which often damages the circuit breaker. Accordingly, circuit breakers of this type usually employ some means of preventing the contact arm from reclosing.
U.S. patent application Ser. No. 09/108,684, filed Jun. 25, 1998, entitled "Rotary Contact Assembly for High Ampere Rated Circuit Breaker" assigned to the same assignee as the present invention describes a rotary type contact assembly using a spring-loaded roller-cam arrangement to lock the contact arm open. In this system, a roller rides along a cam surface on the contact arm and as the contact arm opens under high short circuit conditions the roller follows the cam into a recess in the contact arm. When the roller engages the recess, the contact arm is locked open and prevented from reclosing.
U.S. Pat. No. 5,310,971 entitled "Molded Case Circuit Breaker with Contact Bridge Slowed Down at the End of Repulsion Travel" describes a current limiting rotary type MCCB contact assembly using a spring-loaded pin and cam arrangement to slow the velocity of the contact arm. As the contact arm rotates under magnetic repulsion forces, the energy of the contact arm is absorbed by the springs slowing down the contact arm. When the contact arm reaches it's fully open position, the profile of the cam is such that the contact arm is locked open.
Other examples of current limiting molded case circuit breakers employing contact arm anti-rebound mechanisms can be found in U.S. Pat. No. 5,029,301 entitled "Limiting Circuit Breaker Equipped with an Electromagnetic Effect Contact Fall Delay Device", U.S. Pat. No. 4,263,492 "Circuit Breaker with Anti-Rebound Mechanism", and U.S. Pat. No. 4,611,187 "Circuit Breaker Contact Arm Latch Mechanism for Eliminating Contact Bounce".
Typically, such circuit breakers have a mechanism that under normal usage acts as actuation switch to open the contacts and interrupt the circuit. Under abnormal usage, such as when an over current condition is detected, the mechanism is automatically activated by actuation means well known in the art. This activation creates the necessary opening between the stationary and movable contacts and thus interrupts the current flow. This action of operating the mechanism is inherently slow. As was described above, in current-limiting circuit breakers, the contact arm is allowed to open under the magnetic repulsion forces generated during a short-circuit condition. This repulsion action is independent of the slower methods of opening described above, thus the contact arm opens much faster than the mechanism and it is likely that the contact arm will be locked open before the mechanism has had a chance to react. This situation places the contact arm and rotor/mechanism assemblies in two different positions. Before the circuit breaker can be reset and operated again, the contact arms must be unlocked and allowed to rotate into their normal position with respect to the rest of the assembly.
The typical method for accomplishing this is to use the tripping action of the mechanism associated with automatic actuation. The forces developed by the mechanism are used to unlock the contact arms. In the systems described above, the ability of the lock to operate and stop the rebounding of the arm is proportional to the force developed by the combination of the spring and the cam. Failure to develop a large enough locking force at very high level short circuits will result in the contact arm reclosing. While it is easy to develop an arrangement for producing these large locking forces, there is a limit to how much force the mechanism can produce to unlock the contact arm. Given the size constraints of molded case circuit breakers it is often the mechanism that is the limiting factor in determining at which short circuit level the contact arm will remain locked.
Accordingly, it is considered desirable to have a contact arm locking arrangement that provides a positive lock to prevent contact arm reclosure at any short-circuit level.
It is also desirable to describe a contact arm locking arrangement that where the force required to unlock the contact arm is independent of the force required to lock the arm.
It is also desirable to describe contact arm locking arrangement that allows the contact arm to open as fast as possible while providing a positive locking arrangement which prevents reclosure of the contact arm under high level short circuit conditions.
It is also desirable to provide arrangement for a contact arm locking system for circuit breakers such as those used industrial applications or other applications requiring the protection high amperage circuits.